Most bicycles today are equipped with several chainwheels, generally three in number, and several cogs, generally between 7 and 10. The chainwheels or respectively the cogs have a diameter, and accordingly a number of teeth, that is different from one chainwheel or from one cog to another to allow a choice of gears depending on the chainwheel-cog pair. The highest gear that requires the greatest exertion corresponds to the largest chainwheel and to the smallest cog, whereas the lowest gear that requires the least exertion corresponds to the smallest chainwheel and the largest cog. The user can change gears at will between these two end gears.
It quickly became apparent, however, that a user not used to this plurality of gears could sometimes choose gears incompatible with a long service life of the mechanism. These gears are especially those that cause a major misalignment of the chain, gears such as a large chainwheel/large cog. Moreover, to change from one gear to another without tiring, it is preferable to choose a sequence of gears that causes a progressive increase or decrease of the exertion of the user. At this point, the user has problems in managing this choice of gears. It is often necessary to watch the positioning of its chain on its chainwheels or cogs to effect this choice. Moreover, he sometimes forgets that to change from a cog of a smaller diameter to a cog of a larger diameter reduces the effort to be applied for an identical pedalling rhythm, whereas it is the reverse for the chainwheels, which leads to a selection error.
To avoid all the aforementioned disadvantages, speed changers have been devised.
The first category of speed changer is described especially in patent FR-A-2,530,573. This changer makes it possible to select a chainwheel/cog pair by means of a single control lever. However, the design of the gearshift operating mechanism, the same for the front gearshift and the rear gearshift, is comprised of a cable winding winch and does not allow, for the same direction of actuation of the control lever, actuation of the rear gearshift in two directions in order to preclude a redundant gear or to maintain a relatively constant distance between two successive gears.
A second category of changer that is designed to preclude redundant gears, to prevent overly great misalignments of the chain, and to assign a sequence of gears corresponding to a predetermined speed range, is described especially in European patent EP-A-0,727,348, international applications WO 95/03208 and WO 91/15393 and French patent application FR-A-2,750,669.
This changer thus makes it possible to determine a plurality of cog/chainwheel pairs that will follow one another in a defined manner depending on the actuation of a control device located on the handlebars. This control device is generally located on or in the vicinity of one end of the handlebars, and is preferably comprised of a rotary element such as the well-known rotary grip. This changer makes it possible, when changing a gear, to act solely on one cog, or solely on one chainwheel, or simultaneously on one cog and one chainwheel in order to simultaneously effect a change of the chainwheel and cog in order to optimize the variations of exertion between two successive gears. In these changers, the front or rear gearshift operating mechanism is generally identical. This operating mechanism can be composed especially of cam channels installed on a rotary element. These cam channels can accommodate the ends of the gearshift control cables and in this case are installed in a cylinder as shown by the European patent EP-A-0,727,348. In one variant that is more especially illustrated by international application WO 91/15393, the gearshift control cables are connected to carriages equipped with lugs located in the cam channels of a cylindrical part, the rotation of the cylindrical part causing displacement of the carriages and accordingly the traction or respectively the release of the control cables. A similar principle is disclosed in the U.S. Pat. No. 4,201,095. All these approaches based on one or more cam channels have the disadvantage of inducing a large space requirement of the assembly. As for the speed changer described in patent FR-A-2,750,669, it comprises two different cable operating mechanisms. As in the aforementioned documents, however, these operating mechanisms are permanently actuated in a simultaneous manner. This leads to the necessity of inducing a movement of the sheath relative to the rotary grip. This movement engenders the problem of tightness of the device. One approach, such as the one that is described in patent FR-A-2,758,122 or FR-A-2,758,123, implements cable actuating mechanisms that are again identical and that are especially comprised of a cable winding drum such that simultaneously changing a chainwheel and a cog based on a single control element to avoid a redundant gear is impossible.
Finally, there are likewise speed changers such as those described in international application WO 98/36960 in which the single control grip is a grip that is able to be moved rotationally and translationally so as to act either on the chainwheel alone, or on the cog alone, or on the assembly. Such a solution again requires mental flexibility from the user.